In order to maximize the information content transmitted over a prescribed spectral bandwidth (often measured in bits per Hz of spectral bandwidth), polarization multiplexing (also known as dual-polarization) is being increasingly used with new transmission formats. The underlying idea is that the spectral density (conveniently measured in units of bits/Hz) can be effectively doubled by employing two orthogonally polarized data-carrying signals sharing the same optical signal bandwidth. Normally, these two orthogonally polarized signals are transmitted with approximately the same intensity, rendering the total resultant light effectively unpolarized as seen from a test and measurement instrument having an electronic detection bandwidth significantly lower than the symbol rate of the polarization-multiplexed signal, such as is normally the case with Optical Spectrum Analyzers (OSA).
The Optical Signal-to-Noise Ratio (OSNR) is a conventional measure of the quality of a signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE) noise, which is a spectrally broadband noise source contributed by the optical amplifiers in the link.
Some methods exist for characterizing ASE noise on optical telecommunication signals based on an in-band analysis of the optical signal. Such methods include methods referred to as “polarization-nulling” methods (see J. H. Lee et al., “OSNR Monitoring Technique Using Polarization-Nulling Method”, IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001) as well as variants thereof, and the methods referred to as “differential polarization response” methods (see International Patent Application Publication WO 2008/122123 A1 to Gariepy et al.; and WO 2011/020195 A1 to He et al., both applications being commonly owned by the Applicant). However, such methods are based on the assumption that the signal is generally highly polarized, an assumption that is not valid in the case of polarization-multiplexed signals.
For the case of most polarization-multiplexed signals, the “signal”, as detected on a photodiode having low bandwidth electronics for instance, appears unpolarized, and hence, these above-mentioned in-band OSNR measurement methods cannot be used to reliably provide the OSNR measurement.
In order to measure the noise level or the OSNR of a polarization-multiplexed signal in a multi-channel WDM network, for instance, system manufacturers and operators currently have to resort to turning off the signal at the transmitter in order to measure the noise level and thereby determine the OSNR. A first limitation of this method is that it requires making certain assumptions about the noise variations that occur upon turning off the signal for which the OSNR needs to be measured. The OSNR measurement uncertainty depends, for example, on the number of active channels on the link sharing the same amplified paths. For systems with live traffic, the necessary service interruption resulting from signal turn-off practically limits the use of this technique to a start of life characterization at the time of commissioning a system.
A method of measuring the noise level on polarization-multiplexed signals using an acquired optical spectrum trace of the signal was proposed by Gariepy et al. (see International Patent Application Publication WO 2011/097734 A1, commonly owned by The Applicant and hereby incorporated by reference). This method is based on the knowledge of the spectral shape of the data-carrying signal contribution provided by a reference signal. Based on this knowledge, the signal contribution and the ASE noise contribution that otherwise appear as merged together on the optical spectrum trace, may be mathematically discriminated from one another. Knowledge of the spectral shape of the signal contribution may come from the acquisition of a reference signal taken, for example, at a different point, generally upstream, along the same optical communication link (i.e. the reference signal originates from the same optical transmitter), where the OSNR is known or where the signal can be considered free of ASE noise. This method assumes that, within the optical signal bandwidth, the spectral shape of the signal does not significantly change along the communication link. The signal contribution of such a reference signal is therefore spectrally representative of the signal contribution of the signal-under-test.
However, spectral deformations mostly induced by Non-Linear Effects (NLE) have become more frequent in the case of new deployments using polarization multiplexing, because optimum performance in Bit Error Rate (BER) is obtained by increasing the power beyond the linear regime of optical fibers, non-linear effects therefore arising. As a consequence, signals are subjected to NLE-induced spectral deformations which impact the method proposed by Gariepy et al. Furthermore, aside from the ASE-noise level, non-linear effects now also significantly affect the overall system performance in terms of BER (see Vacondio et al, “On nonlinear distorsions of highly dispersive optical coherent systems”, Optics Express, Vol. 20, No. 2, pp. 1022-1032 (2012)). Signal quality therefore cannot be assessed based only on the conventionally measured ASE noise level because proper performance indicators should also account for NLE-induced distortions to which a spectral deformation of the signal-under-test is correlated.
There is therefore a need for a method to determine quality parameters characterizing polarization-multiplexed signals subject to NLE-induced spectral deformation.